1. Field of the Invention
The present invention relates generally to compositions and methods for specifically targeting pathogens and cells. More particularly, the present invention relates to the use of hybrid molecules including a receptor-binding moiety and an effector moiety for altering the antigenic character of or delivering drugs to pathogens anc cells.
The primary defense mechanism of man and other vertebrates against pathogenic infection is the immune system. The immune response includes two separate pathways to deal with invasion by a foreign substance. The first pathway, referred to as the "humoral response," relies on antibody molecules to bind directly to pathogen to trigger a series of events (the complement cascade or the binding of macrophages and other leukocytes) to eliminate the pathogen from the body. The second immune pathway, referred to as the "cellular response," relies on T-cell recognition of an antigenic region on the pathogen, again leading ultimately to elimination of the pathogen from the system. Both the humoral and cellular responses thus rely on antigenic recognition of the pathogen in order to kill the pathogen and protect the host.
While the immune response is an exquisite and effective protective mechanism against a wide variety of pathogens, there are certain pathogens which evade both pathways of the immune response by changing their cell surface antigens sufficiently rapidly so that they are not recognized by the antibodies which have been elicited during earlier stages of infection. Pathogens with such an ability to evade the immune response include viruses, such as the influenza virus, papilloma viruses, picornaviruses, polyoma virus, and rhinoviruses; bacteria, such as Escherichia coli and Vibrio cholerae; and protozoa, such as Entamoeba histolytica, Trypanosoma cruzii, Plasmodium knowlesi, P. vivax and the like.
Most or all pathogens initiate infection by binding to a surface ligand on the cell being infected. For example, the pathogen may possess a lectin receptor which is able to specifically bind to a carbohydrate ligand on the cell to be infected. To prevent infection, it has been proposed to inhibit initial attachment of the pathogen using drugs which block binding of the pathogen to the cells which are subject to infection. While such drugs can be effective, high dosages may be required to block all available binding receptors on the pathogen. Moreover, the drugs are passive and do not enhance the killing and elimination of the pathogen from the host. Another class of blocking agents include soluble polypeptide receptors, such as soluble CD4 used to inhibit binding of HIV-1 to T-cells. The use of soluble polypeptide receptors has not generally been successful, perhaps due to degradation of the polypeptides after they are administered to a patient.
Sialic acids, derivatives of N-acetyl neuraminic acid (NeuAc), are carbohydrate groups found terminating cell-surface glycoproteins and glycolipids. Glycosides of NeuAc are often utilized by pathogens as an attachment point to cells prior to infection. The use of sialic acid analogs as drugs directed towards the influenza virus has been proposed. The use of O-linked glycosides as potential viral inhibitors, however, is severely limited because of the presence of the neuraminidase enzyme on the virus. This enzyme cleaves the glycosidic bond of NeuAc giving rise to the free sugar which does not inhibit viral attachment. Therefore, a stable non-hydrolyzable analog of sialic acid promises to be useful as an antiviral drug.
For these reasons, it would be desirable to provide improved compositions and methods for enhancing a host's immune response against pathogenic infection, particularly against pathogenic infection by organisms capable of altering their antigenic appearance over time. It would be particularly desirable to provide compositions which are able to target a pathogen and provide at least one invariant antigenic determinant so that a host's immune response can target the pathogen based on the invariant determinant. The compositions will desirably be small, preferably being less than 3 kilodaltons (kD), more preferably being less than 2 kD, and most preferably being less than 1 kD in order to increase their survival time after administration to the host. The compositions should further be substantially free from non-specific binding so that they target the immune response solely against the desired pathogenic organism. The compositions will preferably not themselves be destroyed by the immune response so that individual molecules may successively bind more than one pathogen to reduce the dosage required. It will further be desirable to administer compositions which elicit a secondary or memory response against an antigen against which the host has been previously sensitized.
2. Description of the Background Art
Soluble hybrid molecules, designated immunoadhesins, comprising the gp120-binding domain of CD4 glycoprotein attached to portions of the Fc region of IgG are described in Capon et al. (1989) Nature 337:525-531. See also European Patent Application 0 314 317. Use of the immunoadhesins for treatment of acquired immunodeficiency syndrome (AIDS) is proposed. Hybrid receptors comprising the ligand-binding domain of a receptor, such as a growth factor receptor, attached to a heterologous reporter polypeptide, such as an enzyme are described in U.S. Pat. No. 4,859,609, to Dull and Ullrich. Schultz and Shokat (1991) J. Am. Chem. Soc. 13:1861 describe the use of CD4-nitrophenol conjugates to target anti-DNP antibodies against HIV-1. Sharon and Lis (1989) Science 246:227-234 describe the nature of some pathogenic receptors (lectins) which bind to cell surface carbohydrates to initiate infection. Win compounds are described in Badger et al. (1988) Proc. Natl. Acad. Sci. 85:3304-3308. Win compounds bind to rhinoviruses and inhibit uncoating of virus (which is necessary for infection). The synthesis of sialic acid analogs intended for use as drugs is reported in Sauter et al. (1989) Biochemistry 28:8388 and Whitesides (1991) J. Am. Chem. Soc. 113:686-687.